1. Field
This patent specification relates to a method and apparatus for supplying power, and more particularly to a power supply method and apparatus capable of effectively eliminating an overshoot voltage occurring at a voltage change.
2. Discussion of the Background
Currently, as a power supply circuit for a power supply such as a battery included in a mobile device such as a mobile phone and a digital camera, for example, a non-isolated switching regulator having an inductor (hereinafter referred to as a switching regulator) is used because of high efficiency and smallness in size of the switching regulator.
The switching regulator, however, generates an overshoot voltage due to a circuit configuration thereof, when the switching regulator increases an output voltage. Particularly, when the switching regulator is powered up, a relatively large overshoot voltage is generated. For this reason, as shown in FIG. 1, a conventional switching regulator includes a soft-start circuit employing a method of gradually increasing the output voltage. FIG. 1 is a circuit diagram of an exemplary conventional switching regulator. The switching regulator in this case is a step-down type switching regulator configured to step down an input voltage (hereinafter referred to as a step-down switching regulator). The switching regulator may also be a step-up type switching regulator configured to step up the input voltage (hereinafter referred to as a step-up switching regulator).
The switching regulator 100 of FIG. 1 includes an input terminal IN, an output terminal OUT, a switching transistor Ma, a PWM (pulse width modulation) control circuit 101, an inductor La, a capacitor Ca, a flywheel diode Da, output voltage detecting resistors Ra and Rb, a reference voltage generation circuit 102, a capacitor Cb, a resistor Rc, a switch SW1, and a comparator CMPa. The flywheel diode is also referred to as a freewheeling diode. The switching regulator 100 is connected to a load 110.
In the switching regulator 100, an input source voltage (hereinafter referred to as an input voltage) Vin is input in the input terminal IN, and an output power voltage (hereinafter referred to as an output voltage) Vout is output from the output terminal OUT. The PWM control circuit 101 controls switching of the switching transistor Ma. The switching transistor Ma controls outputting of the input voltage Vin. The inductor La and the capacitor Ca store and discharge energy of the input voltage Vin. The output voltage detecting resistors Ra and Rb detect the output voltage Vout. The reference voltage generation circuit 102 generates and outputs a reference voltage Vref. The comparator CMPa compares the reference voltage Vref with a divided voltage Vd obtained by dividing the output voltage Vout at the output voltage detecting resistors Ra and Rb. The resistor Rc, the capacitor Cb, and the switch SW1 form a time constant circuit to gradually increase the reference voltage Vref at power-up of the switching regulator 100 for applying the reference voltage Vref to the comparator CMPa.
An exemplary soft-start circuit included in the conventional switching regulator 100 is then specifically described. The switch SW1 is turned on at power-up of the switching regulator 100. Then, the capacitor Cb is charged with the reference voltage Vref via the resistor Rc. As a result, a voltage Va at a noninverting input terminal of the comparator CMPa gradually increases, as indicated in a time chart of FIG. 2. Since the output voltage Vout from the switching regulator 100 is proportional to the reference voltage Vref, the output voltage Vout also increases gradually, as observed in the time chart. Accordingly, the overshoot voltage on power up can be prevented. Japanese Laid-Open Patent Publication No. 2000-102243 describes a power supply apparatus using a power supply control IC (integrated circuit) to gradually raise the output voltage without generating the overshoot voltage.
The conventional soft-start circuit, however, does not operate after the switching regulator 100 has been powered up. Therefore, an adverse overshoot voltage is generated when the output voltage Vout is further increased after the power-up of the switching regulator 100. Furthermore, as the output voltage Vout rapidly increases, a capacity component connected to the output terminal is rapidly charged. As a result, an excessively large amount of current is output from the power supply circuit, although a time in which the current is output is relatively short. Accordingly, there arise such problems as a noise-triggered operational error and a failure or deterioration of a device such as the load 110 and the switching transistor Ma caused by the excessively large amount of current sent to the device.